Non-threaded and rotaxane-type threaded wheel-axle assemblies consisting of dinickel(II) metallomacrocycle and dibenzylammonium axle

Rotaxanes are typically prepared using covalent bonds to trap a wheel component onto an axle molecule, and rotaxane-type wheel-axle assembly using only noncovalent interactions has been far less explored. Here we show that a dinickel(II) metallomacrocycle forms two different types of wheel-axle assemblies with a dibenzylammonium axle molecule based only on noncovalent interactions. The non-threaded assembly was obtained by introduction of Ni2+ into the macrocycle before the complexation with the axle molecule (metal-first method). The non-threaded assembly was in rapid equilibrium with each of the components in solution. The threaded assembly was obtained by introduction of Ni2+ after the formation of a pseudorotaxane from the non-metalated wheel and the axle molecule (axle-first method). The threaded assembly was not in equilibrium with the dissociated species even though it was maintained only by noncovalent interactions. Thus, formation of one of the non-threaded and threaded wheel-axle assemblies over the other is governed by the assembly pathway.

Mechanical bonding of rigid MORFs using a tetratopic rotaxane

The preparation of highly rigid cobalt(II)- and copper(II)-organic frameworks incorporating a tetralactam [2]rotaxane as a ligand is described. The interlocked ligand is functionalized with two pairs of carboxylate groups placed at each counterpart, thus limiting its dynamics within the crystal. The solid structure of the metal-organic rotaxane frameworks showed different, unprecedented polycatenation modes of grids, depending on the employed metal, providing great rigidity to the structures. This rigidity has been evaluated by using single crystal X-ray diffraction analyses of the cobalt(II)-organic frameworks embedded in different solvents, observing that the lattices remain unchanged. Thus, this research demonstrates that rigid and robust materials with permanent porosity can be achieved using dynamic ligands.

Formation of polyrotaxane crystals driven by dative boron-nitrogen bonds

The integration of mechanically interlocked molecules (MIMs) into purely organic crystalline materials is expected to produce materials with properties that are not accessible using more classic approaches. To date, this integration has proved elusive. We present a dative boron-nitrogen bond-driven self-assembly strategy that allows for the preparation of polyrotaxane crystals. The polyrotaxane nature of the crystalline material was confirmed by both single-crystal x-ray diffraction analysis and cryogenic high-resolution low-dose transmission electron microscopy. Enhanced softness and greater elasticity are seen for the polyrotaxane crystals than for nonrotaxane polymer controls. This finding is rationalized in terms of the synergetic microscopic motion of the rotaxane subunits. The present work thus highlights the benefits of integrating MIMs into crystalline materials.

Light-responsive rotaxane-based materials: inducing motion in the solid state

Light-responsive rotaxane-based solid-state materials are ideal scaffolds in order to develop smart materials due to the properties provided by the mechanical bond, such as control over the dynamics of the components upon application of external stimuli. This perspective aims to highlight the relevance of these materials, by pointing out recent examples of photoresponsive materials prepared from a rotaxanated architecture in which motion of the counterparts and/or macroscopic motion of the interlocked materials are achieved. Although further development is needed, these materials are envisioned as privileged scaffolds which will be used for different advanced applications in the area of molecular machinery.

Colossal negative thermal expansion in a cucurbit[8]uril-enabled uranyl-organic polythreading framework via thermally induced relaxation

It is an ongoing goal to achieve the effective regulation of the thermal expansion properties of materials. In this work, we propose a method for incorporating host-guest complexation into a framework structure and construct a flexible cucurbit[8]uril uranyl-organic polythreading framework, U₃(bcbpy)₃(CB8). U₃(bcbpy)₃(CB8) can undergo huge negative thermal expansion (NTE) and has a large volumetric coefficient of -962.9 × 10^-6 K^-1 within the temperature range of 260 K to 300 K. Crystallographic snapshots of the polythreading framework at various temperatures reveal that, different from the intrinsic transverse vibrations of the subunits of metal-organic frameworks (MOFs) that experience NTE via a well-known hinging model, the remarkable NTE effect observed here is the result of a newly-proposed thermally induced relaxation process. During this process, an extreme spring-like contraction of the flexible CB8-based pseudorotaxane units, with an onset temperature of ∼260 K, follows a period of cumulative expansion. More interestingly, compared with MOFs that commonly have relatively strong coordination bonds, due to the difference in the structural flexibility and adaptivity of the weakly bonded U₃(bcbpy)₃(CB8) polythreading framework, U₃(bcbpy)₃(CB8) shows unique time-dependent structural dynamics related to the relaxation process, the first time this has been reported in NTE materials. This work provides a feasible pathway for exploring new NTE mechanisms by using tailored supramolecular host-guest complexes with high structural flexibility and has promise for the design of new kinds of functional metal-organic materials with controllable thermal responsive behaviour.

Metal-organic rotaxane frameworks constructed from a cucurbit[8]uril-based ternary complex for the selective detection of antibiotics

Herein we report two 2D layered metal-organic rotaxane frameworks (MORFs), WUST-1 and WUST-2, constituted by a ternary host-guest complex based on cucurbit[8]uril (CB[8]) and an (E)-1-methyl-4-[4-(pyridin-4-yl)styryl] pyridinium (G1) ligand, and different metal ions and auxiliary linkers. Both MORFs are stable in water and highly fluorescence emissive, and can selectively sense nitrofurazone with low detection limits.

Solvent-Dependent Self-Assembly of a Pillar[5]arene-Based Poly-Pseudo-Rotaxane Linked and Threaded by Silver(I) Trifluoroacetate: A Double Role

In the supramolecule area, the fabrication of a new concept called polyrotaxanes or poly-pseudo-rotaxanes remains challenging. We herein report the formation of a poly-pseudo-rotaxane in which the same salt-type guest serves both linking and threading in the resulting structure. The combination of A1/A2-thiopyridyl pillar[5]arene (L) and silver(I) trifluoroacetate in CHCl₃/CH₃OH afforded a one-dimensional (1D) poly-pseudo-rotaxane. In this structure, to our surprise, the AgCF₃CO₂ guest not only links the di-armed L ligands via an infinite -L-Ag-L-Ag- arrangement but also threads into a pillar[5]arene cavity in a dimer form, (AgCF₃CO₂)₂. In contrast, the same reaction in CH₂Cl₂/CH₃OH yielded a simple 1D coordination polymer because an included CH₂Cl₂ molecule in the pillar[5]arene cavity prevents the threading of the silver(I) trifluoroacetate guest. Comparative ¹H- and ¹⁹F-NMR studies support the solvent-dependent poly-pseudo-rotaxane formation at a lower concentration of L.

Anion-Coordination-Driven Assembly: From Discrete Supramolecular Self-Assemblies to Functional Soft Materials

Anion templated assembly of supramolecular systems has been extensively explored in previous reports, whereas anions serve only as an auxiliary and spectator role. With the development of anion coordination chemistry in recent years, anion coordination-driven assembly (ACDA) has emerged as a new strategy for the construction of supramolecular self-assemblies. Anions are proved to exist as the main actors in the construction of supramolecular architectures, i. e., serve as the coordination center. This Review will focus on the recent progress in anion-coordination-driven assembly of discrete supramolecular architectures, such as helicates, polyhedrons and polygons, and the various applications of 'aniono'-systems. At the end of this Review, we highlight current challenges and opportunities for future research of anion-coordination-driven self-assembly.

Organo-macrocycle-containing hierarchical metal-organic frameworks and cages: design, structures, and applications

Developing hierarchical ordered systems is challenging. Using organo-macrocycles to construct metal-organic frameworks (MOFs) and porous coordination cages (PCCs) provides an efficient way to obtain hierarchical assemblies. Macrocycles, such as crown ethers, cyclodextrins, calixarenes, cucurbiturils, and pillararenes, can be incorporated within MOFs/PCCs and they also endow the resultant composites with enhanced properties and functionalities. This review summarizes recent developments of organo-macrocycle-containing hierarchical MOFs/PCCs, emphasizing applications and structure-property relationships of these hierarchically porous materials. This review provides insights for future research on hierarchical self-assembly using macrocycles as building blocks and functional ligands to extend the applications of the composites.

Controlling dynamics in extended molecular frameworks

Molecular machines are essential dynamic components for fuel production, cargo delivery, information storage and processing in living systems. Scientists have demonstrated that they can design and synthesize artificial molecular machines that operate efficiently in isolation - for example, at high dilution in solution - fuelled by chemicals, electricity or light. To organize the spatial arrangement and motion of these machines within close proximity to one another in solid frameworks, such that useful macroscopic work can be performed, remains a challenge in both chemical and materials science. In this Review, we summarize the progress that has been made during the past decade in organizing dynamic molecular entities in such solid frameworks. Emerging applications of these dynamic smart materials in the contexts of molecular recognition, optoelectronics, drug delivery, photodynamic therapy and water desalination are highlighted. Finally, we review recent work on a new non-equilibrium adsorption phenomenon for which we have coined the term mechanisorption. The ability to use external energy to drive directional processes in mechanized extended frameworks augurs well for the future development of artificial molecular factories.

Photoresponsive Metal-Organic Frameworks as Adjustable Scaffolds in Reticular Chemistry

The easy and remote switching of light makes this stimulus an ideal candidate for a large number of applications, among which the preparation of photoresponsive materials stands out. The interest of several scientists in this area in order to achieve improved functionalities has increase parallel to the growth of the structural complexity of these materials. Thus, metal-organic frameworks (MOFs) turned out to be ideal scaffolds for light-responsive ligands. This review is focused on the integration of photoresponsive organic ligands inside MOF crystalline arrays to prepare enhanced functional materials. Besides the summary of the preparation, properties and applications of these materials, an overview of the future outlook of this research area is provided.

Mechanically interlocked molecules in metal-organic frameworks

Mechanically interlocked molecules (MIMs) have great potential in the development of molecular machinery due to their intercomponent dynamics. The incorporation of these molecules in a condensed phase makes it possible to take advantage of the control of the motion of the components at the macroscopic level. Metal-organic frameworks (MOFs) are postulated as ideal supports for intertwined molecules. This review covers the chemistry of the mechanical bond incorporated into metal-organic frameworks from the seminal studies to the latest published advances. We first describe some fundamental concepts of MIMs and MOFs. Next, we summarize the advances in the incorporation of rotaxanes and catenanes inside MOF matrices. Finally, we conclude by showing the study of the rotaxane dynamics in MOFs and the operation of some stimuli-responsive MIMs within MOFs. In addition to emphasising some selected examples, we offer a critical opinion on the state of the art of this research field, remarking the key points on which the future of these systems should be focused.

Controllable photomechanical bending of metal-organic rotaxane crystals facilitated by regioselective confined-space photodimerization

Molecular machines based on mechanically-interlocked molecules (MIMs) such as (pseudo) rotaxanes or catenates are known for their molecular-level dynamics, but promoting macro-mechanical response of these molecular machines or related materials is still challenging. Herein, by employing macrocyclic cucurbit[8]uril (CB[8])-based pseudorotaxane with a pair of styrene-derived photoactive guest molecules as linking structs of uranyl node, we describe a metal-organic rotaxane compound, U-CB[8]-MPyVB, that is capable of delivering controllable macroscopic mechanical responses. Under light irradiation, the ladder-shape structural unit of metal-organic rotaxane chain in U-CB[8]-MPyVB undergoes a regioselective solid-state [2 + 2] photodimerization, and facilitates a photo-triggered single-crystal-to-single-crystal (SCSC) transformation, which even induces macroscopic photomechanical bending of individual rod-like bulk crystals. The fabrication of rotaxane-based crystalline materials with both photoresponsive microscopic and macroscopic dynamic behaviors in solid state can be promising photoactuator devices, and will have implications in emerging fields such as optomechanical microdevices and smart microrobotics.

Macrocycle-Based Solid-State Supramolecular Polymers

Supramolecular polymers, generated by connecting monomers through noncovalent interactions, have received considerable attention over the past years, as they provide versatile platforms for developing diverse aesthetically pleasing polymeric structures with promising applications in a variety of fields, such as medicine, catalysis, and sensing. In the development of supramolecular polymers, macrocyclic hosts play a very important role. Benefiting from their abundant host-guest chemistry and self-assembly characteristics, macrocycles themselves or their host-guest complexes can self-assemble to form well-ordered supramolecular polymeric architectures including pseudopolyrotaxanes and polyrotaxanes. The integration of these topological structures into supramolecular polymeric materials also imbues them with some unforeseen functions. Current interest in macrocycle-based supramolecular polymers is mostly focused on the development of supramolecular soft materials in solution or gel-state, in which the dynamic nature of noncovalent interactions endows supramolecular polymers with a wealth of "smart" properties, such as multiresponsiveness and self-repair capabilities. While preparation of macrocycle-derived supramolecular polymers in the solid state is a relatively challenging but intriguing prospect, they are an important part of the field of supramolecular polymers. On one hand, the construction of macrocycle-based solid-state supramolecular polymers enables us to obtain new materials with novel properties and functions such as mechano-responsiveness. On the other hand, the molecular structures and arrangements in these materials are well-identified by X-ray crystallography techniques, offering a direct visual representation of the supramolecular polymerization process. The analysis of the role of noncovalent interactions in these architectures allows us to design more sophisticated and elegant supramolecular polymers in a highly rationalized and controllable manner. This Account serves to summarize the research progress on macrocycle-based solid-state supramolecular polymers (MSSPs), including the contributions toward this field made by our group. For constructing MSSPs, the key point is to control noncovalent interactions. Thus, in this Account, we primarily classify these MSSPs by different noncovalent interactions involved to connect the monomers, including metal-ligand interactions, host-guest interactions, π···π stacking, and halogen bonding. These noncovalent interactions are highly associated with the structures and functions of the resultant MSSPs. For instance, using metal-ligand interactions as driving forces, metal clusters can be introduced in MSSPs which afford systems with solid-state luminescence or proton conduction properties; supramolecular polymerization using macrocycle-based host-guest interactions can modulate the molecular arrangement of some specific molecules in the solid state, which further influences their solid-state properties; π···π stacking interactions and halogen bonding give chemists more choice to design MSSPs with various elements. The role of macrocyclic hosts in MSSPs is also revealed in these descriptions. Finally, the remaining challenges are identified for further development of future prospects. We hope that this Account can inspire new discoveries in the realm of supramolecular functional systems and offer new opportunities for the construction of supramolecular architectures and solid-state materials.

Rotaxane-branched radical dendrimers with TEMPO termini

The precise synthesis of novel rotaxane-branched radical dendrimers Gn-TEMPO (n = 1-3) with up to 24 TEMPO radicals as termini was successfully achieved, from which nanoparticles with a good longitudinal relaxivity were further prepared, thus making them potential candidates as promising contrast agents for magnetic resonance imaging.

Supramolecular polymer networks crosslinked by crown ether-based host-guest recognition: dynamic materials with tailored mechanical properties in bulk

Supramolecular polymer networks (SPNs) based on host-guest recognition have attracted much research attention to develop smart supramolecular materials. However, these researches mainly focus on SPNs in solution or in gel...

A series of entangled MOFs constructed from flexible dipyridyl piperazine and rigid dicarboxylate: interpenetration, self-penetration, and polycatenation

By employing a flexible dipyridyl piperazine and a rigid linear dicarboxylate, four MOFs with different entangled structures involving a 3D inclined polycatenane and a 3D self-penetrated framework incorporating cyclic [3]catenane were isolated.

Impact of Proximity Effect on Uranyl Coordination of Conformationally Variable Weakly-Bonded Cucurbit[6]uril-Bipyridinium Pseudorotaxane

To explore the proximity effect in uranyl coordination of weak-bonded cucurbit[6]uril(CB[6])-bipyridinium ligands, a new pseudorotaxane precursor C7BPCN3@CB[6] containing 1, 1'-(heptyl-1,7-diyl)bis(3-cyanopyridin-1-ium) bromide (C7BPCN3) with elongated alkyl chains and meta-substituted cyano groups,...

Ensembles from silver clusters and cucurbit[6]uril-containing linkers

In this work, organic supramolecular linkers involving cucubit[6]urils CB[6] and N,N'-hexamethylene-bis(pyrazinyl hexafluorophosphate) (BPHF@CB[6]) were utilized to assemble dodenuclear silver chalcogenolate clusters into three one-dimensional (1D) materials under different synthesis conditions. These three crystal structures of CB[6]-based sliver cluster-organic rotaxane frameworks were well resolved, and their emission properties were investigated systematically. This construction strategy involving organic supramolecular linkers gives a new methodology for cluster-assembled materials with intriguing structural and functional properties.

Dual Electroactivity in a Covalent Organic Network with Mechanically Interlocked Pillar[5]arenes

The synthesis and characterization of a polyrotaxanated covalent organic network (CON) based on the association between the viologen and pillar[5]arene (P[5]OH) units are reported. The mechanical bond allows for the irreversible insertion of n-type redox centers (P[5]OH macrocycles) within a pristine structure based on p-type viologen redox centers. Both redox units are active on a narrow potential range and, in water, the presence of P[5]OH greatly increases the electroactivity of the material.

Synthesis and Characterization of Metallo-Supramolecular Polymers Based on Benzodipyrrolidone

A simple route to the preparation of benzodipyrrolidone (BDP) based monomeric building blocks containing 2,2':6',2″-terpyridines is reported from a common precursor 4'-(4-pinacolatoboronphenyl)-2,2':6',2″-terpyridine via Suzuki coupling reaction. Self-assembly polymerization with ruthenium (II) ions under mild conditions yielded a series of novel metallo-supramolecular polymers with weak donor-acceptor (D-A) structures based on benzodipyrrolidone. The structure of the bridge connected BDP with terpyridine have a significant impact on the wavelength and intensity of the intramolecular charge transfer (ICT) absorption peak. The resulting metallo-polymers exhibited strong double absorption bands around 315 nm and 510 nm involved in π-π* transitions and ICT or metal to ligand charge transfer (MLCT) absorption bands. The forming of D-A structure and coordination with ruthenium (II) ions is favorable to narrow the energy gap and the energy gaps of the resulting metallo-supramolecular polymers are 2.01 and 1.62 eV, respectively.

Translational dynamics of a non-degenerate molecular shuttle imbedded in a zirconium metal-organic framework

A new [2]rotaxane molecular shuttle linker based on the binding of a 24-crown-8 ether macrocycle at a benzimidazole recognition site was synthesised. The shuttling dynamics of the linker were studied in solution and the structure confirmed by X-ray crystallography. A multivariate Zr(iv) MOF, UWDM-11, containing the new MIM linker and primary linker tetramethylterphenyldicarboxylate was synthesised and the translational motion of the molecular shuttle studied in the solid state. The use of a 13C enriched MIM linker allowed the dynamics of both activated and mesitylene-solvated UWDM-11 to be elucidated by VT 13C CPMAS SSNMR. The incorporation of mesitylene into the pores of UWDM-11 resulted in a significant increase in the barrier for thermally driven translation of the macrocycle.

Further investigations of the crystal-to-crystal phase transition of a [2]pseudorotaxane composed of ferrocene-terminated dialkylammonium and dibenzo[24]crown-8-ether

A pseudorotaxane with different or mixed counter anions undergoes thermal phase transition, which is related to the interaction between the counter anion and macrocyclic component.

Coordination-driven assembly of actinide-organic polyrotaxanes involving crown ether macrocycles

Using macrocyclic DB24C8 as a new kind of host molecule for a pseudorotaxane ligand, actinide-linked crown ether-based coordination polyrotaxanes, UCER-1 and UCER-2, that are linked by two different uranyl nodes, have been synthesised for the first time.

pH-Responsive Host-Guest Complexations Between a Water-Soluble Pillar[6]arene Dodecyl-Ammonium Chloride and Aromatic Sulfonic Acids

In the present work, new host-guest binding motifs based on a water-soluble pillar[6]arene dodecyl-ammonium chloride (CP6) with two aromatic sulfonic acids in aqueous media were fabricated. In accordance with the integrated results of ¹H NMR, 2D NOESY, and florescence titration experiments, it was demonstrated that the host-guest binding of CP6 with the two aromatic sulfonic acids in aqueous solution not only has high binding constants but also has pH-responsiveness.

Statistical Mechanical Model of Gas Adsorption in a Metal-Organic Framework Harboring a Rotaxane Molecular Shuttle

Metal-organic frameworks (MOFs) are modular and tunable nanoporous materials with applications in gas storage, separations, and sensing. Integrating flexible/dynamic, gas-responsive components into MOFs can give them unique or enhanced adsorption properties. Here, we explore the adsorption properties that could be imparted to a MOF by a rotaxane molecular shuttle (RMS) in its pores. In the unit cell of an RMS-MOF, a macrocyclic wheel is mechanically interlocked with a strut of the MOF scaffold. The wheel shuttles between stations on the strut that are also gas adsorption sites. At a level of abstraction similar to the seminal Langmuir adsorption model, we pose and analyze a simple statistical mechanical model of gas adsorption in an RMS-MOF that accounts for (i) wheel/gas competition for sites on the strut and (ii) gas-induced changes in the configurational entropy of the shuttling wheel. We determine how the amount of gas adsorbed, the position of the wheel, and the differential energy of adsorption depend on temperature, pressure, and the interactions of the gas and wheel with the stations on the strut. Our model reveals that, compared to a rigid, Langmuir material, the chemistry of the RMS-MOF can be tuned to render gas adsorption more or less temperature sensitive and to release more or less heat upon adsorption. The model also uncovers that, if gas-wheel competition for a station is fierce, temperature influences the position of the wheel differently depending on the amount of gas adsorbed.

Exploring the dynamics of Zr-based metal-organic frameworks containing mechanically interlocked molecular shuttles

Zr(iv) metal-organic frameworks (MOFs) UiO-68 and PCN-57, containing triphenylene dicarboxylate (TPDC) and tetramethyl-triphenylene dicarboxylate (TTDC) linkers, respectively, were doped with an H-shaped tetracarboxylate linker that contains a [2]rotaxane molecular shuttle. The new MOFs, UWDM-8 and UWDM-9, contain a [2]rotaxane crossbar spanning the tetrahedral cavities of the fcu topology while the octahedral cavities remain empty. ¹³C solid-state NMR (SSNMR) spectra and solution ¹H NMR spectra verified that the [2]rotaxanes were included as designed. Variable-temperature (VT) cross polarization (CP) magic-angle spinning (MAS) ¹³C SSNMR was used to explore the translational motion of the macrocyclic ring in both MOFs. The SSNMR results clearly show that the structure of the linker (TPDCvs.TTDC) affects the shuttling rate of the macrocyclic ring, although questions remain as to how rotation of the central phenylene unit of the strut might also affect the motion of the macrocycle.

Synthetic strategies towards mechanically interlocked oligomers and polymers

Mechanically interlocked molecules have fascinated chemists for decades. Initially a tantalising synthetic challenge, interlocked molecules have continued to capture the imagination for their aesthetics and, increasingly, for their potential as molecular machines and use in materials applications. Whilst preliminary statistical attempts to prepare these molecules were exceedingly inefficient, a raft of template-directed strategies have now been realised, providing a vast toolbox from which chemists can access interlocked structures in excellent yields. For many envisaged applications it is desirable to move away from small, discrete interlocked molecules and turn to oligomers and polymers instead, either due to the need for multiple mechanical bonds within the desired material, or to exploit an extended scaffold for the organisation and arrangement of individual mechanically interlocked units. In this tutorial-style review we outline the synthetic strategies that have been employed for the synthesis of mechanically interlocked oligomers and polymers, including oligo-/polymerisation of (pseudo)interlocked precursors, metal-organic self-assembly, the use of orthogonal template motifs, iterative approaches and grafting onto polymer backbones.

N-Heterocyclic Carbene Copper(I) Rotaxanes Mediate Sequential Click Ligations with All Reagents Premixed

We have prepared NHC-Cu^I complexes with a rotaxane structure and used them as sterically sensitive catalysts for one-pot sequential copper-catalyzed azide/alkyne cycloadditions in solutions containing all of the coupling partners premixed in unprotected form. Most notably, a photolabile and sterically encumbered complex first catalyzed the coupling of a less bulky azide/alkyne pair; after removing the protective macrocyclic component from the rotaxane structure, through irradiation with light, the exposed dumbbell-shaped NHC-Cu^I complex catalyzed the second click reaction of a bulkier azide/alkyne pair. Using this approach, we obtained predominantly, from a single sealed pot, a bis-triazole product (84 %) from a mixture of two sterically distinct azides and a diyne.

Regulating the Structures of Self-Assembled Mechanically Interlocked Moleculecular Constructs via Dianion Precursor Substituent Effects

Substituent effects play critical roles in both modulating reaction chemistry and supramolecular self-assembly processes. Using substituted terephthalate dianions (p-phthalic acid dianions; PTADAs), the effect of varying the type, number, and position of the substituents was explored in terms of their ability to regulate the inherent anion complexation features of a tetracationic macrocycle, cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene) (referred to as the Texas-sized molecular box; 1⁴⁺), in the form of its tetrakis-PF₆^- salt in DMSO. Several of the tested substituents, including 2-OH, 2,5-di(OH), 2,5-di(NH₂), 2,5-di(Me), 2,5-di(Cl), 2,5-di(Br), and 2,5-di(I), were found to promote pseudorotaxane formation in contrast to what was seen for the parent PTADA system. Other derivatives of PTADA, including those with 2,3-di(OH), 2,6-di(OH), 2,5-di(OMe), 2,3,5,6-tetra(Cl), and 2,3,5,6-tetra(F) substituents, led only to so-called outside binding, where the anion interacts with 1⁴⁺ on the outside of the macrocyclic cavity. The differing binding modes produced by the choice of PTADA derivative were found to regulate further supramolecular self-assembly when the reaction components included additional metal cations (M). Depending on the specific choice of PTADA derivatives and metal cations (M = Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Gd³⁺, Nd³⁺, Eu³⁺, Sm³⁺, Tb³⁺), constructs involving one-dimensional polyrotaxanes, outside-type rotaxanated supramolecular organic frameworks (RSOFs), or two-dimensional metal-organic rotaxane frameworks (MORFs) could be stabilized. The presence and nature of the substituent were found to dictate which specific higher order self-assembled structure was obtained using a given cation. In the specific case of the 2,5-di(OH), 2,5-di(Cl), and 2,5-di(Br) PTADA derivatives and Eu³⁺, so-called MORFs with distinct fluorescence emission properties could be produced. The present work serves to illustrate how small changes in guest substitution patterns may be used to control structure well beyond the first interaction sphere.

Syntheses of cucurbit[6]uril-based metal-organic rotaxane networks by the anion regulation strategy and their proton conduction properties

Two new cucurbit[6]uril (CB[6])-based metal-organic rotaxane networks (MORNs) were successfully obtained by tuning the coordination sphere of metal copper clusters. Compounds 1 and 2 exhibited relatively high proton conductivity at 85 °C and 97% relative humidity (RH), providing great promise for fuel cell electrolyte materials.

Introducing Seven Transition Metal Ions into Terpyridine-Based Supramolecules: Self-Assembly and Dynamic Ligand Exchange Study

In coordination-driven self-assembly, 2,2':6',2″-terpyridine (tpy) has gained extensive attention in constructing supramolecular architectures on the basis of ⟨tpy-M-tpy⟩ connectivity. In direct self-assembly of large discrete structures, however, the metal ions were mainly limited to Cd(II), Zn(II), and Fe(II) ions. Herein, we significantly broaden the spectrum of metal ions with seven divalent transition metal ions M(II) (M = Mn, Fe, Co, Ni, Cu, Zn, Cd) to assemble a series of supramolecular fractals. In particular, Mn(II), Co(II), Ni(II), and Cu(II) were reported for the first time to form such large and discrete structures with ⟨tpy-M-tpy⟩ connectivity. In addition, the structural stabilities of those supramolecules in the gas phase and the kinetics of the ligand exchange process in solution were investigated using mass spectrometry. Such a fundamental study gave the relative order of structural stability in the gas phase and revealed the inertness of coordination in solution depending on the metal ions. Those results would guide the future study in tpy-based supramolecular chemistry in terms of self-assembly, characterization, property, and application.

Self-assembly of a porous metallo-[5]rotaxane

A dynamic rotaxane ligand self-assembles with palladium(ii) ions to form a metallo-[5]rotaxane with a porous cage at its core.

A [13]rotaxane assembled via a palladium molecular capsule

Molecules that are the size of small proteins are difficult to make. The most frequently examined route is via self-assembly, and one particular approach involves molecular nanocapsules, where ligands are designed that will enforce the formation of specific polyhedra of metals within the core of the structure. Here we show that this approach can be combined with mechanically interlocking molecules to produce nanocapsules that are decorated on their exterior. This could be a general route to very large molecules, and is exemplified here by the synthesis and structural characterization of a [13]rotaxane, containing 150 metal centres. Small angle X-ray scattering combined with atomistic molecular dynamics simulations demonstrate the compound is intact in solution.

Photo- and Redox-Driven Artificial Molecular Motors

Directed motion at the nanoscale is a central attribute of life, and chemically driven motor proteins are nature's choice to accomplish it. Motivated and inspired by such bionanodevices, in the past few decades chemists have developed artificial prototypes of molecular motors, namely, multicomponent synthetic species that exhibit directionally controlled, stimuli-induced movements of their parts. In this context, photonic and redox stimuli represent highly appealing modes of activation, particularly from a technological viewpoint. Here we describe the evolution of the field of photo- and redox-driven artificial molecular motors, and we provide a comprehensive review of the work published in the past 5 years. After an analysis of the general principles that govern controlled and directed movement at the molecular scale, we describe the fundamental photochemical and redox processes that can enable its realization. The main classes of light- and redox-driven molecular motors are illustrated, with a particular focus on recent designs, and a thorough description of the functions performed by these kinds of devices according to literature reports is presented. Limitations, challenges, and future perspectives of the field are critically discussed.

Spontaneous Supramolecular Polymerization Driven by Discrete Platinum Metallacycle-Based Host-Guest Complexation

Considerable progress in platinum metallacycle-based supramolecular polymerization has promoted the fabrication and application of supramolecular materials. However, despite recent advances, supramolecular polymers constructed through platinum metallacycle-based host-guest complexation remain rare because of the dynamics of platinum metallacycles. Here, we achieve linear supramolecular polymerization via platinum metallacycle-based host-guest complexation by following the design rule of suppressing the dynamics of the metallacycles. The establishment of the platinum metallacycle-based host-guest system and the realization of this type of supramolecular polymerization are expected to open opportunities for platinum metallacycle-based functional materials.

Syntheses and magnetic properties of high-dimensional cucurbit[6]uril-based metal-organic rotaxane frameworks

Three new high-dimensional cucurbit[6]-based metal-organic rotaxane frameworks [Co₂(PR43)(BDC)₂Cl₂]·4H₂O (1), [Co₂(PR43)(BTC)₂]·6H₂O (2) and [Co₂(PR43)(BPT)₂]·20H₂O (3) were obtained via the hydrothermal synthesis method. Compound 1 comprised a two-dimensional layered structure, while compounds 2 and 3 exhibited three-dimensional pillared structures. All the compounds showed good thermal stabilities. Furthermore, the magnetic properties of compounds 1-3 were also investigated in detail.

Biological Macrocycle: Supramolecular Hydrophobic Guest Transport System Based on Nanodiscs with Photodynamic Activity

A biogenic macrocycle-based guest loading system has been developed by the self-assembly of membrane scaffold protein and phospholipids. The resulting 10 nm level transport system can increase the solubility of hydrophobic photodynamic agent hypocrellin B in aqueous medium and exhibited a cellular internalization capacity with substantial photodynamic activity.

Acid/base- and base/acid-switchable complexation between anionic-/cationic-pillar[6]arenes and a viologen ditosylate salt

Two new host-guest complexes between water-soluble anionic pillar[6]arene (WP6) or cationic pillar[6]arene (CP6) and a viologen ditosylate salt G·2TsO were constructed, among which one formed from WP6 and G2+ ions can be controlled by the sequential addition of an acid and a base (HCl and NaOH, respectively), whereas the other fabricated from CP6 and TsO- ions can be switched through the sequential addition of basic and acidic reagents (NaOH and HCl, respectively).